1. Field of the Invention
The invention herein relates to the removal of contaminants from fluid streams. More particularly it relates to the production of substantially contaminant free supercritical and liquid carbon dioxide (CO2) fluid streams.
2. Description of the Prior Art
A supercritical fluid is a fluid which is in a state above its critical temperature and critical pressure where gas and liquid phases resolve into a single medium, in which density can vary widely without a phase transition. This allows, for instance, substances which normally act as solvents primarily for inorganic or polar substances to also become efficient solvents for organic or nonpolar materials. Because of these unique properties, supercritical fluids are used in a wide variety of industries, particularly for solvent extraction; examples include polymer and pharmaceutical manufacturing, food processing, environmental processes and precision cleaning in manufacture of products where high purity and cleanliness is required, such as semiconductor manufacturing, including removal or organic photoresist materials. For the most part, achieving supercriticality requires rasing gaseous or liquid compounds to quite high temperatures and pressures.
In contrast, a compound which finds numerous uses in its supercritical state is carbon dioxide, which reaches supercritical state under moderate conditions at a critical point (CP) of 31.3xc2x0 C. (88.3xc2x0 F.) and 74 bar (1070 psi). The supercritical region for carbon dioxide is shown in the upper right hand portion of the P/T graph of FIG. 7. Another fluid phase, liquid carbon dioxide, exists at temperatures in the range shown substantially in the center of the P/T graph of FIG. 7. Essentially the liquid region is bounded by the supercritical region above 31.3xc2x0 C. (88.3xc2x0 F.) and 74 bar (1070 psi), the pressure/temperature curve between the liquid and gaseous phase regions defining distillation Of CO2, running from the triple point (TP) at xe2x88x9256.6xc2x0 C. (xe2x88x9269.9xc2x0 F.) and 5.2 bar (75.1 psi) to the critical point, and the liquid/solid curve defining solidification and running from the triple point at substantially constant temperature and increasing pressures. Carbon dioxide is therefore liquid at moderate pressures and temperatures (including xe2x80x9croom temperaturexe2x80x9d of 25xc2x0 C. [77xc2x0 F.] and about 68 bar [985 psi]).
For brevity herein the term xe2x80x9cfluid carbon dioxidexe2x80x9d or xe2x80x9cfluid CO2xe2x80x9d will be used as the comprehensive term to refer to both liquid and supercritical carbon dioxide, where the text discusses information applicable to both. Where one or the other phase is expressly intended, it will be so identified. Further, supercritical carbon dioxide will also sometimes be referred to as xe2x80x9cSC CO2xe2x80x9d.
In the manufacture of semiconductors and wafers, it is critical that at each stage of manufacture the materials be extremely clean. The presence of any significant amount of contamination will usually render the final product unusable. Manufacturing specifications therefore commonly require that concentrations of contaminants in each stage of manufacturing be maintained in the sub-ppb (parts per billion) range. A common class of contaminant in the manufacture of semiconductors and wafers is hydrocarbons, which occur as residues from use of hydrocarbon solvents earlier in the manufacturing process, from hydrocarbon skin oils left by workers handling the materials, from lubricants used in the compressors and pumps which move the high pressure gases through the production chambers, and from lubricants used on other manufacturing equipment.
Fluid carbon dioxide, especially SC CO2, has been found to be particularly useful for cleaning of semiconductors and wafers, where it is used in the form of xe2x80x9csnowxe2x80x9d or solid carbon dioxide flakes. The snow is formed when the fluid carbon dioxide is xe2x80x9cflashedxe2x80x9d or sprayed at high flow rate from nozzles into the cleaning chamber. In one mode of decontamination, the carbon dioxide flakes traveling at high speed impact against the contaminants on the surface of the semiconductor or wafer and cause the contaminants to be ejected into the moving carbon dioxide gas stream. In another mode the fluid carbon dioxide absorbs the contaminants from the semiconductor or wafer surface, in a manner akin to solvent extraction. The solid carbon dioxide snow then sublimes into the gas stream and is carried away for an environmentally safe recovery.
Of course for this cleaning system to work effectively the fluid carbon dioxide itself must be contaminant free. While there have been prior systems to decontaminate carbon dioxide, such prior art systems (such as distillation and cryogenic processes) have all been cumbersome, time-consuming and of limited effectiveness. Further, many processes focus on decontaminating the carbon dioxide as a gas prior to its being raised to liquid or supercritical conditions. This creates a potential for recontamination during the conversion to the fluid state, since oils and lubricants used in the compressors and pumps used to achieve the liquid and supercritical pressures often contaminate the CO2 as it is being compressed. To make fluid carbon dioxide cleaning as efficient as possible, it is important to reduce contaminants in the fluid carbon dioxide itself, and, consistent with the other specified system contaminant levels, particularly to reduce them to a ppb level. Prior art systems to have been unable to reach this level with fluid carbon dioxide on a consistent basis or in an economically viable manner.
Many processes exist to decontaminate gases by passing them through beds of conventional zeolites, silica, alumina and other oxides, metals, etc. Commercial products produced by the assignee of this patent application, Aeronex, Inc., of San Diego, Calif., have incorporated high silica content zeolites for the removal of water from corrosive gas streams; patents have been applied for for such products and the decontamination methods they employ. None of these prior art systems, however, has been used to decontaminate a gaseous compound while it is in its liquid or supercritical state.
In addition to the Aeronex product and method mentioned above, zeolites have also been used in other contaminant removal processes, but primarily as carriers or substrates for various impregnated metal getters or dehydrating or decontaminating catalysts. In this regard they have merely been substitutes for conventional silica, alumina and carbon substrates.
Further, none of the prior art processes has had the ability to decontaminate fluid CO2 on a continuous basis. To accomplish that, one must have the ability to regenerate some of the decontaminant capacity while operating the remaining capacity for decontamination. A process for continuous removal of water and CO2 from specialty gases in a two-vessel system is shown in U.S. Pat. No. 5,833,738, but that process is not self-regenerating, since it uses nitrogen from a source outside the system for the regeneration, and passes purified gas from the purification vessel to the regeneration vessel only for a short initial period to equilibrate the vessels.
We have now developed a unique and highly effective process for the removal of contaminants from fluid (liquid and supercritical) carbon dioxide down to essentially a 1 ppb concentration. Intermediate levels which can readily be reached are 100 ppb and 10 ppb. This decontamination process can be operated for long periods of time, since the critical material used is not susceptible to degradation in the carbon dioxide liquid phase and supercritical phase temperature and pressure regimes. The process also provides for self-regeneration of the deactivated bed of one vessel with purified gas from the operating bed of the other vessel. This permits continual production of purified fluid CO2 from the process, by alternating use of the vessels with the zeolite bed of one vessel decontaminating the fluid CO2 while the zeolite bed of the other vessel is being regenerated.
We have discovered that high silica zeolites, particularly high silica mordenite and its analogs, can be used very effectively as catalysts to reduce the contaminant level in fluid carbon dioxide to 100 ppb or lower, and in many cases down to at least 50 ppb, 10 ppb and in some cases to about 1 ppb. The high silica zeolites are preferable used as a porous bulk material, but can be in the form of coating on a substrate.
The hydrocarbons and water which are the main contaminants in the incoming fluid CO2 are absorbed in the zeolite bed, resulting in a product stream of purified fluid CO2. This product stream may be the initial input stream to the process utilizing the fluid CO2 for cleaning, or it may be a recycle stream being returned to the cleaning process for reuse after the fluid CO2 has been purified. In the first case, the feed stream to the purification system will be a stream of fresh (but not decontaminated) fluid CO2, while in the second case the feed stream will be the outlet product of the cleaning process, i.e., the fluid CO2 cleaner stream contaminated with the materials picked up during the cleaning process.
The principal contaminants which are removed from the fluid carbon dioxide are various hydrocarbons, which may have been absorbed by the carbon dioxide from a number of different sources (the specific sources not being significant for the purposes of this invention), because of the affinity for absorption of hydrocarbons by carbon dioxide. Their removal is important for two reasons. First, since the fluid carbon dioxide (whether as fresh feed or recycle) will be used in the precision cleaning process to remove contaminants, many of which will be hydrocarbons, its removal efficiency is enhanced if its initial hydrocarbon contaminant content is minimal. Second, when the fluid carbon dioxide is flashed to form the cleaning snow, hydrocarbon contaminants will be separated from the carbon dioxide and can deposit as contaminants on the semiconductor or wafer which is to be cleaned.
Zeolites are a class of synthetic and natural minerals having an aluminosilicate tetrahedral framework, ion-exchangable large cations, and 10%-20% loosely held water molecules which permit reversible dehydration without significant alteration in the molecular structure. They are often referred to as xe2x80x9cmolecular sievesxe2x80x9d because of their ability to separate gaseous and liquid molecules on the basis of molecular size. The metal cations present are primarily sodium and calcium, but may also include various alkali metal or alkaline elements such as potassium, strontium and barium. To be suitable for the present invention, the zeolites must have the water removed and also the alumina content must be reduce to a point where the silica is the predominant component of the zeolite structure. Particularly preferred in this invention is a high silica mordenite.
In the preferred xe2x80x9chigh silicaxe2x80x9d zeolite structure of the present invention, the SiO2:Al2O3 ratio of the zeolite will be at least 20:1, preferably will be at least 90:1, and more preferably will be at least 300:1 or higher. (Ratios of 500:1, 2000:1 and higher would be even better, and while there are not currently any commercial zeolites which have that high a ratio, their use in this invention is contemplated when they become available.) It is believed that the critical aspect which determines suitability of a particular zeolite for the present invention is the ability to undergo alumina removal without significant alteration in the zeolite structure or metal cation content, so that the large pore size (4 xc3x85-20 xc3x85) remains after the alumina is removed. For instance, with the preferred material, silica mordenite, the mordenite structure (whether natural or synthetic) is considered to be quite suitable for decontamination of a supercritical carbon dioxide stream, because it is a good adsorbent and also because of its structural stability in the flowing gas stream, so that the pores do not collapse under pressure.
While we do not wish to be bound by any specification of a mechanism, it is believed that the system operates because the hydrocarbons and other contaminants preferentially bind to the hydrophilic active sites within the zeolite pores, blocking the carbon dioxide from binding so that it passes though to be used in the thus-purified state.
Therefore, in one broad embodiment, the invention is of a method for removing contaminants from a stream of fluid carbon dioxide which comprises contacting the fluid carbon dioxide with a quantity of high silica zeolite for a period of time sufficient to reduce the contaminant content of the fluid carbon dioxide stream to not more than 100 ppb, preferably to not more than 10-50 ppb, and more preferably to not more than about 1 ppb.
The invention also includes operating the method in a manner which allows the system to be self-regenerating. This comprises dividing the zeolite into at least two beds (normally of essentially equal capacity) each placed in its own reaction vessel, operating one bed to decontaminate the contaminated fluid carbon dioxide and simultaneously regenerating the other zeolite bed using a portion of the purified fluid carbon dioxide product from the operating bed. The draw-off stream of the purified fluid CO2 product regenerates the zeolite bed by removing the contaminants which accumulated on the zeolite during the bed""s previous decontamination operation. The regeneration rate is controlled such that the bed is fully regenerated several hours or days before the other bed""s decontamination capacity is reached, so that when the other bed approaches its capacity for adequately decontaminating the contaminated fluid CO2, the contaminated CO2 fluid stream can (within a period of a few seconds) be diverted to the regenerated bed and the decontamination of the fluid CO2 can proceed substantially without interruption. A draw-off stream of a portion of the purified fluid CO2 is passed from the newly decontaminating bed and passed to the other vessel so that the bed whose capacity was reached can be regenerated. By thus alternating operating and regenerating zeolite beds, virtually continuous production of purified fluid carbon dioxide can be achieved and maintained for extended periods of time, generally measured in months and years.
Thus, in another broad embodiment, the invention is of a method for removing contaminants from a stream of fluid carbon dioxide which comprises contacting the fluid carbon dioxide with a quantity of high silica zeolite for a period of time sufficient to reduce the contaminant content of the CO2 fluid stream to not more than 100 ppb, preferably to not more than 10-50 ppb, and more preferably to not more than about 1 ppb, in which the high silica zeolite is disposed in at least two separate quantities within a corresponding number of vessels, the vessels being interconnected such that contaminated fluid carbon dioxide may be directed to one of the vessels for decontamination, with a portion of decontaminated fluid carbon dioxide exiting from that vessel being passed to a second vessel and used to remove accumulated fluid carbon dioxide contaminants from zeolite within the second vessel, such that the zeolite within the second vessel may subsequently be used to decontaminate additional contaminated fluid carbon dioxide.
Also as part of this embodiment are the steps, upon cessation of decontamination in the first vessel and of removal of contaminates from the zeolite in the second vessel, of diverting contaminated fluid carbon dioxide to the second vessel and passing a portion of the purified fluid carbon dioxide product therefrom to the first vessel to remove accumulated contaminants from zeolite therein and thus rejuvenate the first zeolite bed. The repeated alternation of decontaminating and regenerating beds thus permits the decontamination process to continue for extended periods, with no need to provide regeneration from any source outside the system.